1. Field of the Invention
This invention relates to a process for producing a fluorine-containing elastic copolymer, and more particularly to a process for producing a fluorine-containing elastic copolymer with distinguished low temperature characteristics.
2. Description of the Prior Art
Japanese Patent Application Kokai (Laid-open) No. 58-71906 discloses a process for producing a fluorine-containing elastic copolymer by copolymerizing fluorovinylether represented by the following general formula: EQU CF.sub.2 .dbd.CF(OCF.sub.2 CFX).sub.n ORf
with tetrafluoroethylene, etc. The thus obtained fluorine-containing elastic copolymer has ethereal bonds in the main chain and its glass transition temperature is lowered with increasing content of the ethereal bonds (15-50% by mole in terms of fluorovinylether). However, when the copolymerization reaction is carried out by solution polymerization, i.e. the ordinary polymerization procedure, the solution viscosty increases with the progress of the copolymerization reaction, and stirring of the copolymerization system becomes hard to carry out. As a result, dispersion of the resulting copolymers becomes poorer, and it is difficult to conduct control of the heat of polymerization and uniform heating (see Japanese Patent Publication No. 48-41942). In case of emulsion polymerization, on the other hand, the copolymerization reaction must be carried out at a relatively high temperature, and the resulting copolymers often have unstable terminal groups, and separation of the emulsifying agent from the reaction mixture is very difficult to carry out (Japanese Patent Publication No. 48-20788). Thus, there are problems in the copolymerization and desired elastic copolymers are hard to obtain.
U.S. Pat. No. 4,281,092 discloses a process for producing a fluorine-containing elastic copolymer by copolymerizing tetrafluoroethylene with perfluoro(lower alkyl vinyl ether) in the presence of a curing site monomer represented by the following general formula: EQU CF.sub.2 .dbd.CF[OCF.sub.2 CF(CF.sub.3)].sub.m O(CF.sub.2).sub.n CN
However, the lowering of the glass transition temperature of the fluorine-containing elastic copolymer is not satisfactory, even if the copolymerization ratio of perfluoro(lower alkyl vinyl ether) is made as high as possible.